Problem with using an item from inventory

Hi,

so here's the deal:

I followed a tutorial for an inventory. In this tutorial you use the item when clicking on it. I on the other hand would like a submenu to pop up when clicking on an item where you can choose weather to "use", "combine" or "cancel".

So here is the code from the tutorial which works fine.

public class Slot : MonoBehaviour, IPointerClickHandler {

  private Stack<Item> items;
 
  public Sprite slotEmpty;
 
  public Sprite slotHighlighted;
 
  public bool IsEmpty{
      get{return items.Count == 0;}
  }
 
  private GameObject useButtons;
 
  // Use this for initialization
  void Start () {
      items = new Stack<Item> ();
      RectTransform slotRect = GetComponent<RectTransform>();
      useButtons = GameObject.Find("Canvas/Use Buttons");
      useButtons.active = false;
  }
  
  // Update is called once per frame
  void Update () {
  
  }
 
  public void AddItem (Item item){
      items.Push (item);
 
      ChangeSprite (item.spriteNeutral, item.spriteHighlighted);
  }
 
  private void ChangeSprite(Sprite neutral, Sprite highlight)
  {
      GetComponent<Image> ().sprite = neutral;
 
      SpriteState st = new SpriteState ();
      st.highlightedSprite = highlight;
      st.pressedSprite = neutral;
 
      GetComponent<Button> ().spriteState = st;
  }
 
  private void UseItem()
  {
      if (!IsEmpty) {
          items.Pop ().Use ();
          useButtons.active = false;
 
          if (IsEmpty) {
              ChangeSprite (slotEmpty, slotHighlighted);
              Inventory.EmptySlots++;
          }
      }
  }
  
  /*private void Benutzen()
  {
      useButtons.active = true;
  }
 
  public void UseButon()
  {
      UseItem ();
  }*/
 
  #region IPointerClickHandler implementation
 
  public void OnPointerClick (PointerEventData eventData)
  {
      if (eventData.button == PointerEventData.InputButton.Left)
      {
          UseItem();
      }
  }
 
  #endregion

}

My code is in the / / thingis. When I activate these lines and chance the "OnPointerClick" command to "Benutzen();" it gives me a NullReferenceException on line 16.

Does anyone know what to do?

Thanks in advance ;)